Because direct evaluation of the damage to somatic or germ cells of humans that may cause cancer or hereditary disease is not technically possible, and because epidemiological studies are limited and difficult to interpret, we propose to use the normal human diploid fibroblast in culture as a model for the systematic investigation of mutagenic changes that occur in human cells in vivo. Sound quantitative and qualitative data describing the range of mutagenic responses of cells from normal individuals in the human population are needed as base line control values for further investigation of high-risk groups for studies of the mechanisms of mutagenesis and the role of somatic mutation in human cancer, and for evaluation of the validity of short-term test systems for making risk estimates in screens for potential mutagens and carcinogens. The proposed research aims (a) to perform dose-response mutagenesis determinations for three mutagens on independent, randomly selected normal strains of skin-derived fibroblasts, as a first attempt to collect sufficient data to estimate the range of mutability in the human population and (b) to perform the same determinations on lung-derived strains from the same individuals, as a first attempt to collect sufficient data to compare the mutagenic responses of fibroblasts from two different human tissues. The validity of the analysis of variance in mutability between individuals and between tissues is dependent upon the degree of accuracy of the estimates of individual mutation frequencies and the number of individual strains tested. Our goal is to design experiments that ensure the observation of statistically adequate numbers of colonies for each of the measurements that enter into the final estimate of induced mutation frequency and to examine the mutagenic response of an adequate sample of normal fibroblast strains. Our previous MNNG results demonstrate that statistical measurements with the human fibroblast mutagenesis system are technically feasible. We also propose to characterize the mutants induced by three different mutagens and to further develop the mutagenesis system by more precisely defining the length of phenotypic expression time needed following mutagen treatment.